1. Field of the Invention
The present invention relates to a system and apparatus for the preparation of low concentration primary standard mixes. The standard mixes of the present invention are utilized for calibrating analytical detectors, such as mass spectrometers, and for analysis of emissions from combustion chambers and process tools.
2. Description of Related Art
The demand for low concentration primary standard gas mixtures in the range of (100-1000 parts per billion (ppb)), is increasing in a number of industries. Of particular interest for environmental emissions testing is a mixture of nitric oxide in nitrogen, as environmental regulations become increasingly strict.
The need for supplying gas with UHP purity levels has led the industry to develop analytical techniques for measuring gas impurities. Advances in gas analysis instrumentation in the same range of impurities as would be found in typical gas analysis instrumentation has led to increase demand for low concentration primary standard mixes employed as calibration gases.
Currently, low concentration primary mixes are prepared by two methods as described by G.O. Nelson, Gas Mixtures Preparation and Control, Lewis Publishers, Ann Arbor, Mich. (1992). One is a static mixture, where a volume of the desired mixture is generated and then contained in a cylinder at either low or high pressures. The mixture is subsequently utilized for the particular application. Another is the dynamic mixture where the components of interest are introduced into a stream of purified diluent gas at essentially atmospheric pressure, and the desired concentration is generated. The mixture is thereafter consumed as a calibration gas for an analytical instrument.
Over the years a number of methods have been devised to control the dynamic addition of the components of interest to the diluent gas. In this regard, Leggett et al in U.S. Pat. No. 5,214,952 discloses a calibration device utilizing a series of highly accurate mass flow controllers to provide rapid delivery of ultra high calibration gas mixtures, and sample gas, to a gas analyzer at elevated temperatures.
Ridgeway et al in U.S. Pat. No 5,661,225 discloses a system for the dynamic dilution of a high concentration analyte containing gas for calibrating analytical detectors. The calibration systems described in Leggett et al and Ridgeway et al include permeation tubes and mass flow controllers for the dynamic addition of the components of interest to the diluent gas.
Some of the drawbacks associated with the static mixture related art includes the number of sequential dilutions necessary for each component added to arrive at the standard gas mixture. For example, the uncertainty in the final concentration increases with the number of dilutions. As such, in the related art it is necessary to have a minimum of three dilutions to generate a primary standard at concentrations below one part per million. Multiple dilutions can also deleteriously contribute to the contamination of the process as exposure to the ambient atmosphere is increased. In addition, the multiple dilution method requires considerable capital expenditure, as a skilled operator is required to monitor and intervene in the process.
To overcome the disadvantages of the related art, it is an object of this invention to provide a system and apparatus for producing static mixtures of low concentration (i.e., 10 ppb to 1000 ppb) primary standard mixtures.
It is another object of this invention, to utilize a permeation device, as a precise metering device to dispense minor components directly into a cylinder for a predetermined period of time, thereby allowing to weight traceable back to a National Institute of Standards Technology (NIST) standard.
Other objects and aspects of the present invention will become apparent to one skilled in the art on a review of the specification and claims appended hereto.